1. Field of the Invention
The present invention relates to an optical communications network, and in particular to a repeating installation used in a synchronous optical communications network.
Recently, as a hierarchy providing a wideband service, an SDH (Synchronous Digital Hierarchy) has been internationally standardized. A SONET (Synchronous Optical NETwork), which is used in a North America, is one example of such a hierarchy.
2. Description of the Related Art
FIG. 1 shows a block diagram of an example of a SONET transmission system. A repeating installation, ADM (Add/Drop Multiplexer) is provided between two apparatuses which comprise line terminated equipment (LTE) LTE1 and LTE2 which face each other in a point-to-point network arrangement. Each of the line terminated equipment LTE1 and LTE2 forms a higher-order group optical interface (input and output interface) for a multiplexed optical signal OC-N (`OC` being an abbreviation of Optical Container and `N` being an arbitrary number). For example, OC-12 has a transmission rate of 622.08 Mbps. The line terminated equipment LTE1 and LTE2 are connected with each other, via the repeating installation ADM, using two both-way optical transmission circuits L1 and L2. Either circuit (optical fibers) of the two optical transmission circuits L1 and L2 is used as a currently used circuit and the other one is used as a spare circuit. The repeating installation ADM has functions of extraction and insertion of a service signal (terminating function), in addition to functions of regenerative repeating, extraction and insertion for a main signal (terminating function). The service signal contains various monitoring and control information and is used for maintaining and monitoring a telephone line, for maintenance persons to communicate with each other, and SONET transmission system. Such a service signal is transmitted in use of a section overhead SOH or a line overhead LOH contained in a transmission signal.
In a SONET, the section overhead SOH is used for managing a repeating section between repeating installations, and the line overhead LOH is used for managing a multiplex section between line terminated equipment. As will be described later, the section overhead SOH has an arrangement of 3 rows by 9 columns and the line overhead LOH has an arrangement of 5 rows by 9 columns. The section overhead SOH and line overhead LOH are added at the top of a multiplexed digital signal. A path overhead for managing a path between multiplexing apparatuses is also used for maintenance and monitoring.
FIG. 2 shows a block diagram of part of the above-mentioned repeating installation ADM. Interfaces INF1 and INF2 are provided for the transmission circuits L1 and L2 from the line terminated equipment LTE1, respectively. Interfaces INF3 and INF4 are provided for the transmission circuits L1 and L2 from the line terminated equipment LTE2, respectively. In FIG. 2, circuits for extracting overheads SOH and LOH (service signal) in the interfaces IF1, IF2, IF3 and IF4 are indicated, and circuits for inserting the overheads SOH and LOH are omitted. Outputs of the interfaces INF1 and INF2 are integrated through a high impedance control unit C, and similarly outputs of the interfaces INF3 and INF4 are integrated through a high impedance control unit C. The high impedance control unit C of either one of the interfaces INF1 and INF2 is set to be at a high impedance, and the high impedance control unit C of either one of the interfaces INF3 and INF4 is set to be at a high impedance. Accordingly, the overheads SOH and LOH outputted by either one of the interfaces INF1 and INF2 are extracted, and the overheads SOH and LOH outputted by either one of the interfaces INF3 and INF4 are extracted. Because the same signal is transmitted in the currently used circuit and spare circuit, it is enough that the overheads SOH and LOH of the signal is transmitted via either one of the two circuits.
As a repeating installation similar to the above-described repeating installation ADM, there is a regenerator (REG). As shown in FIG. 3, the regenerator REG, similar to the repeating installation ADM, is provided between the line terminated equipment LTE1 and LTE2. Further, the regenerator REG may be provided between the repeating installations ADM. Different from the repeating installation ADM, the regenerator REG does not have the functions of extracting and inserting a main signal and line overhead LOH.
FIG. 4 shows a block diagram of part of the above-described regenerator REG. As shown in the figure, interfaces INF11 through INF14 are provided for respective lines of two transmission circuits. Different from the repeating installation ADM, the regenerator REG does not necessarily perform repeating on both the currently used circuit and the spare circuit. For example, shown in FIG. 5, the regenerator REG performs repeating on either one of the currently used circuit and the spare circuit between the line terminated equipment LTE1 and LTE2, and on either one of the currently used circuit and the spare circuit between the line terminated equipment LTE3 and LTE4, Accordingly, different from an arrangement shown in FIG. 2, the regenerator REG does not have the high impedance control unit C. Specifically, as shown in FIG. 4, the section overheads are extracted from all of the four lines of the two transmission circuits. Further, as described above, the regenerator REG does not extract and cause the line overheads to pass therethrough.
FIG. 6 shows a block diagram of part of a regenerator REG in the related art. The regenerator REG shown in the figure has higher-order group interface units 10, which are provided for the four lines of the two transmission circuits, and overhead processing mechanism 12 which has overhead processing units 12a, 12b, 12c and 12d. Section overheads SOH extracted in the higher-order group interface units 10 are sent to and terminated in the overhead processing units 12a, 12b, 12c and 12d, respectively. Main signals indicated by bold lines in FIG. 6 pass through the higher-order interface units 10 (it being regenerative repeating and the main signals not being terminated there.)
However, in the regenerator in the related art shown in FIG. 6, the overhead processing units 12a through 12d are provided for the respective lines of the transmission circuits and thus prevent the installation from being miniaturized, decreasing power consumption and decreasing cost.